1. FIELD OF THE INVENTION
The present invention relates to an annular light source unit using electrodeless discharge, which has an annular, i.e., circular or polygonal, doughnut-like electrodeless discharge tube which is excited with use of a microwave power to cause production, mainly, of ultraviolet rays, the ultraviolet rays being intended for use in photoengraving, cure of paint or ink, etc., and a method of lighting the same.
2. DESCRIPTION OF THE PRIOR ART
A conventional example of this type of light source unit using electrodeless discharge is disclosed in Japanese Patent Examined Publication No. 59-23613. This example has a slender electrodeless discharge tube in which there is sealed argon or mercury vapor, which is excited with use of a microwave power to cause the occurrence of a discharge phenomenon therein, to thereby cause production of ultraviolet rays.
In the above-mentioned official gazette, another conventional example also is disclosed, which has a spherical electrodeless discharge tube, which is excited with use of a microwave power for causing production of ultraviolet rays by way of a discharge phenomenon similar to that mentioned above.
Japanese Patent Examined Publication No. 55-35825 (U.S. Pat. No. 3,872,349) discloses still another conventional example of this type of light source unit using electrodeless discharge. The electrodeless discharge tube employed in this example is a slender linear one which is semicircular or circular in cross section, or is a small spherical one which serves as a point light source. The microwave power is used to excite the argon or mercury vapor which is sealed in the discharge tube of such shape. This excitation causes the occurrence of a discharge phenomenon in the argon or mercury vapor to cause production of ultraviolet rays.
Among the above-described conventional examples, however, the slender linear electrodeless discharge tube has a drawback in that it is difficult to excite the contents of the tube over an entire length thereof, uniformly, by supply of a microwave power and that, therefore, uniform emission of light over the entire length thereof is impossible.
Conventional electrodeless discharge tube in a spherical shape has also a drawback. A sphere is, in principle, the smallest in surface area of all shapes having an equal volume. Furthermore, the greater the diameter, the smaller the surface area per unit volume. Therefore, an increase in the diameter of the spherical electrodeless discharge tube to increase the amount of light emitted therefrom deteriorates cooling of the sphere and causes it to overheat.